Oil and gas production pipelines may allow multiphase flow, in which more than one fluid is transported in the pipeline. This is advantageous because where more than one fluid is to be transported, only one pipeline is needed and no separation equipment is required. This is particularly advantageous in environments that are difficult to reach, such as sea beds and harsh climates, as the use of only one pipeline greatly reduces capital costs.
The different fluids flowing in the pipeline at the same time have different flow characteristics governed by their differing viscosities and densities. This makes it difficult to characterize multiphase flow. It is important to be able to characterize this with as the flow characteristics have an effect on plugging, corrosion and erosion of the pipeline. Plugging can occur by the formation of waxy deposits in the pipeline. In order to operate a multiphase flow pipeline safely it is therefore imperative to have a good knowledge about the flow characteristics in the pipeline. One of the more important flow characteristics is the phase distribution (sometimes termed flow regime) in the pipeline.
Flow characteristics are typically determined using empirical equations that have been tested using laboratory experiments. However, these equations are limited as they cannot take account of all of the variables that may be present in a working pipeline, such as inclination, variations in flow rate and so one. Furthermore, as the test rigs used to verify the empirical equations have significantly smaller diameters than production pipelines, the validity of the models for large diameter pipes is unknown.
Multiphase flow can also be predicted theoretically using models and equations but these suffer similar limitations to using empirical models.
GB2307047 describes a method of determining volumetric flow rates of gas and liquid in stratified flow. This requires using a flow meter or radioactive tracer to measure the flow of the liquid phase and a flow meter or temperature sensors to measure the flow of the gas phase in the multiphase flow. The system requires installing equipment within a pipeline, which is costly and disruptive.
US 2008/0163692 describes a system using thermal sensor probes for flow analysis in a multiphase pipeline. In this system, constant temperature is applied at different points around the pipeline, and the temperature difference between each heat source and the pipeline wall is measured using several heat sources and associated probes.